U.S. patent number 4,680,974 [Application Number 06/826,315] was granted by the patent office on 1987-07-21 for mass flow meter on the coriolis principle.
This patent grant is currently assigned to Danfoss A/S. Invention is credited to Hans J. Moos, Jens K. Simonsen.
United States Patent |
4,680,974 |
Simonsen , et al. |
July 21, 1987 |
Mass flow meter on the coriolis principle
Abstract
The invention relates to a mass flow meter operable on the
Coriolis principle having two rectilinear juxtaposed parallel
arranged measuring tubes mechanically interconnected at their ends.
An oscillator between the tubes produces opposite oscillatory
movement of the tubes with a harmonic oscillation superimposed on
the fundamental oscillation. Sensors between the tubes on opposite
sides of the oscillator sense relative movement between the tubes
and generate signals corresponding thereto. A circuit responsive to
these signals determines resonant frequencies of the fundamental
and harmonic oscillations and derives therefrom a correcting value
which gives effect to axial stresses in the measuring tubes to
determine a corrected mass flow.
Inventors: |
Simonsen; Jens K. (Nordborg,
DK), Moos; Hans J. (Nordborg, DK) |
Assignee: |
Danfoss A/S (Nordborg,
DK)
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Family
ID: |
6262577 |
Appl.
No.: |
06/826,315 |
Filed: |
February 5, 1986 |
Foreign Application Priority Data
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Feb 15, 1985 [DE] |
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3505166 |
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Current U.S.
Class: |
73/861.357 |
Current CPC
Class: |
G01F
1/8431 (20130101); G01F 1/8495 (20130101); G01F
1/8436 (20130101) |
Current International
Class: |
G01F
1/76 (20060101); G01F 1/84 (20060101); G01F
001/84 () |
Field of
Search: |
;73/861.37,861.38 |
Foreign Patent Documents
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0119638 |
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Sep 1984 |
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EP |
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0165016 |
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Sep 1983 |
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JP |
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Primary Examiner: Goldstein; Herbert
Attorney, Agent or Firm: Easton; Wayne B. Johnson; Clayton
R.
Claims
We claim:
1. A mass flow meter operable on the Coriolis principle,
comprising, two rectilinear juxtaposed parallel arranged tubes
mechanically interconnected at their ends, wye tube fittings
connecting adjacent ends of said tubes to provide for connections
to supply and discharge pipes and for parallel flow through said
tubes, oscillator means between said tubes for producing opposite
oscillatory movement of said tubes with a harmonic oscillation
superimposed on the fundamental oscillation, first and second
sensor means between and at opposite ends of said tubes and spaced
from said oscillator means for sensing relative movement between
said tubes and generating first and second signals corresponding
thereto, means responsive to said first and second signals for
generating a first value of mass flow, frequency determining
circuit means responsive to said first signal for determining the
resonant frequencies of said fundamental and harmonic oscillations
and for deriving therefrom a correcting value which (gives effect
to) is a measure of axial stresses in said measuring tubes and
means for applying said correcting value to said first value to
determine a corrected mass flow.
2. A meter according to claim 1 including a correcting circuit
which forms a quotient from said resonant frequencies of said
fundamental and harmonic oscillations, said correcting value being
a predetermined function of said quotient.
3. A meter according to claim 2 wherein said correcting circuit
includes a store for receiving data of said predetermined function
and generates said correcting value from said determined
quotient.
4. A meter according to claim 1 characterized in that said harmonic
oscillation corresponds to the third harmonic wave.
5. A meter according to claim 1 wherein said oscillater means is
disposed substantially midway between the ends of said tubes, and
said first sensor means being positioned approximately one-fifth
the length of said tubes from one end thereof.
6. A meter according to claim 1 wherein said circuit means includes
exciter circuit means for driving said oscillator means having an
input connected to said first sensor means, said exciter circuit
means having fundamental and harmonic oscillation branches, said
harmonic oscillation branch having a selection filter arrangement
and amplifier means for selecting said resonant frequency of said
harmonic oscillations, and a summation element connected to said
oscillator means which receives amplified signals of both of said
branches.
7. A meter according to claim 6 characterized in that said
summation element is a summation amplifier with AGC regulation.
8. A meter according to claim 6 characterized in that said
fundamental oscillation branch has phase correcting means to set
said exciter circuit means in phase with said first signal, and
said harmonic oscillation branch has phase reversal means to set
said harmonic oscillation branch in phase with said harmonic
oscillations of said first signal.
9. A meter according to claim 6 including a voltagecurrent
transformer connected between said summation element and said
oscillator means.
10. A meter according to claim 6 wherein said selection filter
arrangement includes a band filter with a selection frequency
predetermined by timing pulses, and a pulse generator for
generating said timing pulses having a timing frequency which is a
multiple of the frequency of the harmonic in said harmonic
oscillation branch.
11. A meter according to claim 10 including amplifier means for the
output of said harmonic oscillation branch, said pulse generator
including a phase locking circuit having first and second inputs
and an output, and a 1:N divider between said output and second
input.
12. A meter according to claim 11 having a starting circuit
including logic circuit means connected to said phase locking
circuit, further input means for said summation element, said logic
circuit means transmitting a square signal to said further input
means when said phase locking circuit is under voltage and this
circuit is not yet locked.
13. A meter according to claim 6 wherein said frequency determining
circuit has two frequency signal outputs, and means connecting said
outputs to said fundamental oscillation branch and said harmonic
oscillation branch.
Description
The invention relates to a mass flow meter on the Coriolis
principle, wherein two juxtaposed measuring tubes are mechanically
interconnected at their ends and connected for parallel flow by
means of two tube connectors which are connected at their
non-confronting ends to a supply or discharge passage having a
connector at its end, wherein an oscillator is provided, which sets
the measuring tubes into opposite fundamental oscillations, and
wherein the measuring tubes are associated at a spacing from the
oscillator with sensors for receiving measuring signals from which
the flow of mass can be determined.
In a known meter of this kind U.S. Pat. No. 4,491,025 a cylindrical
container provided at its ends with connectors for the supply and
withdrawal of the medium to be measured and at the middle with
dividing walls carries two tubes bent into U shape communicating
with the interior of the container at both sides of the dividing
walls. The container therefore defines the tube connectors and the
supply and withdrawal passages. The adjacent limbs of the U tubes
are mechanically interconnected near the container with straps
which define the ends of the actual measuring tubes which can be
oppositely oscillated by the oscillator. The oscillator is applied
at the middle of the curved web of the U. The sensors are disposed
at the transition between the curves and the straight limbs of the
tube. The particular mass flow can be determined from the
difference in the phases of the oscillatory motion at both ends of
the U curve. Since the oscillating measuring tubes must have a
certain length but project laterally from the container, the meter
becomes laterally bulky.
The object of the invention is to provide a mass flow meter of the
aforementioned kind that is laterally more compact.
This problem is solved according to the invention in that the
measuring tubes are straight and parallel, that the oscillator
produces a harmonic superimposed on the fundamental oscillation,
and that a frequency determining circuit is provided which
determines from a measuring signal the values of the resonance
frequencies of the fundamental and harmonic oscillations for the
purpose of deriving therefrom a correcting value which takes axial
stresses in the measuring tubes into account to determine a
corrected mass flow.
In this construction, straight measuring tubes are used instead of
bent ones. The lateral extent is therefore small. The measuring
tubes can extend parallel to the conduit in which the meter is
connected. However, since the tube connectors are now widely spaced
from one another, changes in length occur as a result of
temperature fluctuations. If, as is usual, the tube connectors and
connections form a solid unit which is spacially fixed by being
applied to the conduit, the change in length will lead to axial
stresses in the measuring tubes, by which the oscillatory behaviour
is altered and there will be errors in measurement. Axial stresses
can also occur through incorrect clamping of the device and for
other reasons. The axial stresses have different effects on the
fundamental and harmonic oscillations. Consequently, if excitation
is not only by means of a fundamental oscillation but also with a
superimposed harmonic, the size of the axial force can be derived
from the two frequencies and hence also a correcting value for
compensating the measuring error. Thus, despite axial stresses
which are inevitable with temperature changes, the mass flow meter
is adapted to give corrected values of mass flow.
Preferably, a correcting circuit is provided which forms a quotient
from the frequencies of the fundamental and harmonic oscillations,
the correcting value being a predetermined function of said
quotient. The ratio of the two frequencies is a particularly simple
measure of the axial stresses and hence also of the correcting
value. This function could even represent a correcting factor which
can be particularly easily linked with the measuring result.
In particular, the correcting circuit may comprise a store for
receiving date of the predetermined function and automatically make
the correcting value available by reason of the determined
quotient. The store therefore assumes the function of a table or
computing rule. Since the correcting value is given automatically,
it is constantly available.
A particularly simple circuit is obtained if an evaluating circuit
for determining the mass flow from measuring signals received by
two spaced sensors is followed by a multiplication element which is
fed with the correcting value determined from the quotient so as to
determine the corrected mass flow.
With particular advantage, the harmonic oscillation corresponds to
the third harmonic wave. This can readily be excited by the same
position as the fundamental oscillation. In addition, compared with
other harmonics it has the largest amplitude, so that it can be
readily detected if the sensor is suitably placed.
In a preferred form of the invention, the oscillator is disposed
substantially in the middle of the straight measuring tubes and at
least one sensor is disposed at a spacing of 15 to 25%, preferably
about 20%, from the end of the measuring tube. By means of the
central arrangement, the fundamental and third harmonic
oscillations are excited under optimum conditions. The special
position of the sensor ensures that the third harmonic will be
detected near its greatest amplitude and the fundamental
oscillation will likewise be detectd with an adequate
amplitude.
With particular advantage, the oscillator is fed by an exciter
circuit comprising an input connected to a sensor, a fundamental
oscillation branch provided with an amplifier, a harmonic
oscillation branch provided with a selection filter arrangement and
an amplifier, and a summation element which precedes the output and
receives the amplified signals of both branches. With the aid of
the harmonic oscillation branch, the harmonic can be separately
treated and amplified so that it can be added in a predetermined
ratio to the signal of the fundamental oscillation branch. In this
way, one ensures that sufficient excitation energy is available for
the harmonic. Otherwise, the preferably adjustable admixing can be
so selected that evaluation of the phase displacement of the
fundamental frequency for determining the measured flow quantity is
not influenced by the harmonic.
It is favourable if the summation element is a summation amplifier
with AGC (automatic gain control) regulation. The energising power
is therefore so regulated that the measuring signals have a certain
size permitting their evaluation.
In addition, each branch should contain a phase correcting element.
Small correcting values suffice for the fundamental oscillation.
Considerable phase rotations may be necessary for the harmonics,
for example a phase reversal for the third harmonic.
Further, it is advisable for a voltage-current transformer to be
connected between the summation element and oscillator. In this
way, one eliminates phase displacements on account of the
inductance of the coils of the oscillator and measurement errors
associated therewith.
With particular advantage, the selection filter arrangement
comprises a band filter with a selection frequency predetermined by
timing pulses and a pulse generator is provided of which the
frequency is a multiple of the frequency of the harmonic in the
harmonic oscillation branch and is made to follow same. In this
way, one ensures that, despite the changes in the harmonic
occurring with axial stresses, the selection filter arrangement
will always accurately tune its mean frequency to the existing
harmonic frequency. This avoids the phase rotations occuring on
frequency changes with a solis filter.
In particular, the pulse generator may comprise a phase locking
circuit of which the first input is connected by way of a
comparator to a section of the harmonic oscillation branch
following the amplifier and the second input is connected by way of
a 1:N divider to its output. This gives a particularly simple
construction for the pulse generator which depends on the harmonic
frequency.
Further, it is advisable to have a starting circuit in which the
summation element has a further input which receives a square
signal by way of a logic circuit when the first input of the phase
locking circuit is under voltage and this circuit is not yet
locked. This can also initiate excitation of the harmonic so that
phase locking occurs after a short time and the selection filter
can operate normally.
It is also advantageous if the frequency determining circuit is
formed by utilising the exciter circuit and comprises two frequency
signal outputs each connected by way of a comparator to a section
of the fundamental oscillation branch or harmonic oscillation
branch that follows the amplifier. Signals of the frequencies to be
determined are simply obtained at the frequency signal outputs.
An example of the invention will now be described in more detail
with reference to the drawing, in which:
FIG. 1 is a diagrammatic representation of a mass flow meter with
associated circuit;
FIG. 2 shows an embodiment of a sensor;
FIG. 3 shows an embodiment of an oscillator;
FIG. 4 shows the oscillating behaviour of a measuring tube; and
FIG. 5 shows an example of an exciter circuit.
The mass flow meter 1 shown in FIG. 1 comprises two measuring tubes
2 and 3 which are straight and parallel. At their ends, they are
mechanically interconnected by cross-struts 4 and 4a. The measuring
tubes are connected for parallel flow with the aid of two tube
connectors 5 and 6. The passages 7 and 8 serving for supply and
withdrawal are provided at their non-confronting ends with an end
connector 9 or 10. With its connectors 9 and 10, the meter can
therefore be included in a conduit containing the medium to be
measured.
Substantially in the middle of the tubes there is an oscillator 11
comprising a permanent magnet 12 connected to the measuring tube 2
and a drive coil 13 connected to the measuring tube 3. At
substantially equal spacings in front of and behind this
oscillator, there are two sensors 14 and 15 each comprising a
permanent magnet 16 or 17 connected to the measuring tube 2 and an
induction coil 18 or 19. These have a spacing of about 20% of the
measuring tube length from the end of the measuring tube. If a
periodic exciter current I is fed to the oscillator, the two
measuring tubes 2 and 3 will oscillate in opposite senses. By
reason of the oscillating motion, a measuring signal U1 and U2 is
induced in the induction coils 18 and 19 of the sensors 14 and 15
that is in the form of a voltage proportional to the velocity of
the movements of the measuring tubes relatively to each other.
A particularly effective example of a sensor is shown in FIG. 2.
The reference numerals are increased by 100 in relation to FIG. 1.
A permanent magnet 116 magnetised as south pole S and north pole N
next to each other transversely is opposite an induction coil 118
with an axis parallel to the measuring tubes.
A particularly effective example of oscillator 111 is shown in FIG.
3. A permanent magnet 112 likewise magnetised transversely next to
each other as south pole S and north pole N is disposed within a
drive coil 113 consisting of a carrier 120 of non-magnetisable
material.
An exciter circuit 21 to be explained in more detail in conjunction
with FIG. 5 receives the measuring signal U.sub.1 at its input 22
by way of a conduit 23 and delivers the exciter current I to the
oscillator 11 by way of its output conduit 24. The exciter circuit
21 is such that the exciter current brings the measuring tubes into
resonance in regard to the fundamental oscillation F.sub.1 and
their third harmonic F.sub.3,as diagrammatically shown in FIG. 4.
The fundamental oscillation F.sub.1 of each measuring tube occurs
between the full line F.sub.1 and the broken line. The amplitude of
the third harmonic F.sub.3 is considerably less than shown and
superimposed on the fundamental oscillation. The measuring signal
U.sub.1 is fed to the one input 25 and the measuring signal U.sub.2
by way of a conduit 26 to the other input 27 of a phase detector 28
which, by reason of the phase displacement of the fundamental
oscillation in both measuring signals delivers an uncorrected flow
value Q.sub.1 at its output 29. This is based on the known fact
that, by reason of the Coriolis force, the mass of the medium
flowing through the measuring tubes displaces the phase of the tube
oscillations initiated by the oscillator 11 over the tube length.
The phase displacement is most easily determined in that the time
difference between the occurrence of the zero points is found in
both measuring signals U.sub.1 and U.sub.2. This is proportional to
the uncorrected value Q.sub.1 of the mass flow.
By reason of temperature fluctuations or solely its clamping, the
meter clamped in position by its connectors 9 and 10 undergoes
axial loading. The axial stresses caused thereby likewise lead to a
change in the oscillating behaviour, so that the uncorrected
flowQ.sub.1 is in error. For this reason, a part of the exciter
circuit 21 forms a frequency determining circuit 30 which makes
available at the outputs 31 and 32 the determined resonance
frequencies f.sub.1 and f.sub.3 for the fundamental oscillation and
third harmonic. The two frequencies are fed to a correcting circuit
33 which forms a quotient from these frequencies f.sub.1 and
f.sub.3 in a first section 34. By reason of this quotient, a data
stor 35 is given a correcting factor k which is transmitted to a
multiplication element 36. Accordingly, the corrected flow Q.sub.2
=k.times.Q.sub.1 can be indicated in a display unit 37 or otherwise
processed. The upper harmonics are here designated with an ordinate
which is referred to as a fundamental oscillation with the ordinate
1. By reason of the temperature and the cross-section of the
measuring tubes, the resonance frequencies of these oscillations
are not necessarily in a precise whole number relationship to each
other.
The construction of the exciter circuit is evident from FIG. 5.
Together with the measuring tube system, it forms oscillator means
of which the tube system represents the resonance circuit and the
exciter circuit gives the required loop amplification and feedback.
As a result, the system automatically sets itself to the resonance
frequencies of the tube system. It is therefore possible to
resonate the tube system simultaneously with the resonance
frequencies f.sub.1 and f.sub.3 of the fundamental and harmonic
oscillations. The measuring signal U.sub.1 is fed by way of a
pre-amplifier A1 to a fundamental oscillating branch 38 and a
harmonic oscillation branch 39. The fundamental oscillation branch
38 comprises a phase correcting ciruit PC1 and an amplifer A2.
Since the fundamental oscillation in the measuring signal U.sub.1
is substantially in phase with the fundamental oscillation in the
exciter current I, Only a slight correction is necessary in the
phase correcting circuit PC1. The harmonic oscillation branch 39
comprises a high pass filter HPF, a phase correcting circuit PC2, a
selection filter SF and an amplifier A3. The measuring signal
U.sub.1 contains the third harmonic out of phase with the harmonic
in the exciter current I. For this reason, the phase correcting
circuit PC2 effects a phase reversal. The output signal of branch
38 is fed by way of a summation resistor R1 to a summation
amplifier A4 to which there is also fed by way of a summation
resistor R2 the output signal of branch 39 which is tapped at a
potentiometer P1 so as to select the ratio of fundamental
oscillation and harmonic in the output signal in such a way, that
on the one hand a marked third harmonic is present in the measuring
tube but on the other hand the evaluation of the phase position of
the fundamental oscillation is not affected in the phase detector
28. The measuring signal U.sub.1 amplified in the preamplifier A1
is also fed to an automatic amplifying regulator AGC which compares
the amplitude of the amplified measuring signal with a desired
value settable at a potentiometer P2 and, depending thereon, so
regulates the amplification of the summation amplifier A4 that, as
is diagrammatically illustrated by a potentiotmeter P3 in the
return circuit, the measuring signal amplitude corresponds to the
desired value. The output value of the summation amplifier A4 is
fed by way of a voltage-current transformer U/I and a terminal
stage E to the oscillator 11 as current I.
In order that the harmonic, in this case the third, can be filtered
out cleanly, the high pass filter HPF which blocks for lower
frequencies is supplemented by the selection filter SF of which the
mean frequency determining the filtering function is determined by
timing pulses i.sub.t which are produced by a pulse generator 40
and supplied by way of a line 41 at a pulse frequency f.sub.t n
times the harmonic frequency f.sub.3. For this purpose, the one
input 41 of a phase locking circuit PLL is connected by way of a
comparator K1, which herein is functionally equivalent to a Schmidt
trigger to the output of amplifier A3 of the harmonic oscillation
branch 39 and the second input 42 is connected by way of a divider
T to the output 43 of the phase locking circuit. The latter
conventionally consists of the series circuit of a phase
comparator, a low pass filter and a voltage-controlled oscillator.
The pulse frequency f.sub.t is a whole number multiple of the
harmonic frequency f.sub.3. N can for example have the value 64.
With the aid of potentiometers P4 and P5, the selection filter SF
can additionally be set. It is a so-called tracking filter, for
example of type MF 10 by Messrs. National. Because the mean
frequency of the selection filter SF follows the resonance
frequency f.sub.3 of the harmonic, one ensures that the filter is
very accurately tuned to this frequency f.sub.3 i.e. the third
harmonic is amplified whereas all other frequencies are heavily
damped.
A starter circuit A4 comprises a logic circuit 44 with two NAND
elements N1 and N2. The NAND element N2 feeds the summation
amplifier A4 by way of a third summation resistor R3 with randomly
occurring square pulses whenever square pulses are present at the
output 45 of comparator K1 and it is simultaneously indicated by
the occurrence of a signal 0 at a further output 46 of the phase
locking circuit PLL that no phase locking has as yet taken place.
On the other hand, if the signal 1 occurs at output 46 on locking,
i.e. during normal operation, the NAND element N2 remains blocked.
The irregularly occuring square pulses produce an oscillation at
varying frequencies. By reason of the construction of the exciter
circuit 21, the fundamental and third harmonic oscillations will
soon predominate, so that normal operating conditions are rapidly
attained.
In such an exciter circuit 21, the frequency determining circuit 30
can have a very simple construction. The output 31 need merely be
connected by way of a comparator K2 to the output of amplifier A2
in the fundamental oscillation branch 38 and the output 32 to the
output 45 of the comparator K1 of the harmonic oscillation branch
39. Square pulses of resonance frequency f.sub.1 of the fundamental
oscillation will then occur at output 31 and square pulses of the
resonance frequency f.sub.3 of the third harmonic at output 32.
The function for determining the correcting factor k is easily
determined experimentally in the following manner. First, in two
attempts one ascertains the resonance frequencies for the
fundamental and harmonic oscillations in dependence on the axial
force loading the measuring tubes, the axial force preferably being
standardised to Euler's bending force. This shows that both
frequencies change but the resonance frequency of the fundamental
oscillation much more so than that of the harmonic. If one
interlinks these two frequencies in any formula, for example by
forming a ratio, one obtains a clear relationship to the
instantaneous axial loading condition. If in a further test series,
the axial force is varied at constant mass flow, one
obtains--starting from the unloaded condition--a correcting factor
k which depends on the axial force. With the aid of both tests, one
can therefore interlink this correcting factor and the two
resonance frequencies in a function. This function may be stored in
the store 35.
Instead of the correcting factor k, one can use an additive
correcting value if the correcting circuit 33 is fed with the value
for the uncorrected flow Q.sub.1.
To determine the axial force and the correcting value dependent
thereon, one can also use the resonance frequencies of oscillations
other than the fundamental or third harmonic. In particular, one
can use the second harmonic for this purpose but this requires
excitation at a position other than the middle and thus a higher
excitation energy. At higher harmonics, one has to make do with
smaller oscillating amplitudes.
* * * * *